Macrophage-adipocyte interaction: marked interleukin-6 production by lipopolysaccharide

Recent studies suggested macrophages were integrated in adipose tissues, interacting with adipocytes, thereby exacerbating inflammatory responses. Persistent low-grade infection by gram-negative bacteria appears to promote atherogenesis. We hypothesized a ligand for toll-like receptor 4 (TLR4), bacterial lipopolysaccharide (LPS), would further exaggerate macrophage-adipocyte interaction.

RAW264.7 macrophage cell line and differentiated 3T3-L1 preadipocytes were co-cultured using transwell system. As a control, each cell was cultured independently. After incubation of the cells with or without Escherichia coli LPS, tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 production was evaluated.

Co-culture of macrophages and adipocytes with low concentration of Escherichia coli LPS (1 ng/mL) markedly up-regulated IL-6 production (nearly 100-fold higher than that of adipocyte culture alone, p < 0.01), whereas TNF-alpha production was not significantly influenced. This increase was partially inhibited by anti-TNF-alpha neutralizing antibody. Recombinant TNF-alpha and LPS synergistically up-regulated IL-6 production in adipocytes. However, this increase did not reach the level of production observed in co-cultures stimulated with LPS.

A ligand for TLR-4 stimulates macrophages to produce TNF-alpha. TNF-alpha, thus produced, cooperatively up-regulates IL-6 production with other soluble factors secreted either from adipocytes or macrophages in these cells. Markedly up-regulated IL-6 would greatly influence the pathophysiology of diabetes and its vascular complications.